The present application relates generally to organic electroluminescent display, and particularly relates to a passive matrix-driven display based on electroluminescent polymers having a structured matrix of pixels and a structured second electrode, as well as its production.
The graphic depiction of information is becoming increasingly important in our daily life. More and more ordinary items are being provided with display elements, which make it possible to immediately obtain the information needed on site. In addition to the conventional cathode ray tube (CRT), which provides high image resolution but is associated with the disadvantage of high weight and high power consumption, the technology of flat panel displays (FPDs) was developed for use in mobile electronic devices.
The mobility of the devices places high demands on the display to be used. The first requirement to be mentioned in this connection is low weight, which eliminates conventional CRTs from the outset. Minimal depth of construction is another essential criterion. In fact, in many devices a depth of construction of the display of less than one millimeter is required. Because of the limited capacity of conventional or rechargeable batteries in mobile devices, low power consumption of the displays is also necessary. Another criterion is good legibility, even when there is a large angle between the display surface and the viewer, as well as legibility under various ambient light conditions. The capacity to display multi-color or full-color information is also becoming more and more important. And, last but not least, the useful lifetime of the components is also an important pre-requisite for use in various devices. The relative importance of individual criteria for the displays varies depending on the application.
Several technologies have already been developed for some time in the market of flat panel displays, all of which cannot be discussed in detail here. So-called liquid crystal (LC) displays are overwhelmingly dominant. Aside from its cost-effective producibility, low electric power consumption, low weight and low space requirements, the LCD technology also has serious disadvantages. LC displays are not self-emitting and therefore can only be easily read or recognized under especially favorable ambient lighting conditions. In most cases, this creates the need for back-lighting which, however, increases the thickness of the flat panel display many times over. Furthermore, most of the electric power consumption is used for illumination, and higher voltage is needed to operate the light bulbs or fluorescent tubes. This is usually generated from the conventional or rechargeable batteries using “voltage-up converters.” Another disadvantage is the highly limited viewing angle of simple LCDs and the lengthy switching times of individual pixels, which typically comprise several milliseconds and are also highly temperature-dependent. The delayed image development is extremely bothersome, especially when LCDs are used in means of transportation or in video applications.
In addition to LCDs, other flat display technologies, such as vacuum fluorescence displays or inorganic thin layer electroluminescence displays, also exist. However, these technologies have either not yet reached the necessary technical level of development or, because of high operating voltages or production costs, are only partially suited for use in portable electronic devices.
Since 1987, displays based on organic light emitting diodes (OLEDs) have become known. They do not have the disadvantages mentioned above. Because of self-emissiveness, the need for back-lighting is eliminated, which substantially reduces space requirements and electric power consumption. The switching times fall within the range of one microsecond and are only slightly temperature-dependent, which makes the use of these displays possible for video applications. The viewing angle is almost 180°. Polarization films such as those required with LC displays are generally not needed, so that greater brightness of the display elements can be achieved. Another advantage is the ability to use flexible and non-planar substrates, as well as simple and cost-effective production.
For OLEDs, two technologies exist which differ in terms of the nature and processing of the organic materials. On the one hand, low molecular weight organic materials, such as hydroxiquinoline-aluminum-III salt (Alq3) can be used, which are generally applied to the appropriate substrate by means of thermal vapor deposition. Displays based on this technology are already commercially available and currently used primarily in automobile electronics. However, because the production of these components is associated with many process steps under a high vacuum, high investment and maintenance costs as well as relatively low rates of throughput make this technology disadvantageous.
For these reasons, an OLED technology has been developed since 1990 that utilizes, as organic materials, polymers that are wet-chemically applied from a solution onto the substrate. The vacuum steps needed to generate the organic layers are eliminated in this technology. Typical polymers are polyaniline, PEDOT (Bayer Co.), poly(p-phenylene-vinylene), poly(2-methoxy-5-(2′-ethyl)-hexyloxy-p-phenylene-vinylene), or polyalkylfluorene, as well as many derivatives thereof.